Structural and Mechanical Characterizations of Top Dross in a Molten Zinc Bath

Abstract

In a molten zinc bath in a continuous galvanizing line (CGL), top dross particles crystallize as Fe-Al-Zn intermetallic compounds. These particles easily adhere to the steel sheets causing surface defects. Therefore, controlling the top dross particles is a key issue. The present study focused on the structural and mechanical characterizations of top dross particles using an electron probe micro analyzer, X-ray diffraction, electron back scattering diffraction, Vickers hardness measurement and nano-indentation measurement. The following results were obtained: (1) The crystal structure of top dross particles Fe₂Al₅Zn_x having Fe: 37~38 wt%, Al: 44~45 wt% and Zn: 18~19 wt% belongs to the orthorhombic system with a lattice constant of a=7.61 Å, b=6.48 Å and c=4.23 Å. The a axis of Fe₂Al₅Zn_x becomes shorter, while the b and c axes become longer compared to those of binary Fe₂Al₅. (2) The top dross particles with the faceted interface were postulated to coarsen by the mechanism of the anisotropic interface energy between the top dross particles and molten Zn as a driving force rather than by the aggregation mechanism. (3) The hardness and the elastic modulus of the top dross particles are the lowest in the [001] direction like Fe₂Al₅, and are lower than those of Fe₂Al₅. (4) The fracture toughness of top dross particles is approximately 1.1 MPa·m^1/2, which is slightly lower than that of Fe₂Al₅.

EBSD (a) image quality map, (b) phase map, and inverse pole figure maps of (c) Fe₂Al₅Zn_x phase dross particles and (d) Zn.